1. Field of the Invention
The present invention relates to an improvement of a gear drive device for transmitting a driving force of a traction motor to an axle in a track of a railway motor car, which consists of a larger gear fixed on the axle and a smaller gear fixed on the traction motor.
2. Description of the Prior Art
As a railway motor car having a motive power, there are an electric locomotive, an electric car, a Diesel electric locomotive which has a generator driven by a Diesel engine and traction electric motors driven by electric power generated by the generator, and so on. A conventional gear drive device applied to an electric locomotive is described below. FIG. 6 is a side view showing a constitution of an electric locomotive, schematically. In FIG. 6, the electric locomotive 10 comprises a body 11, bogies 12, a pantograph 15 for collecting an electric power from an overhead wire 17. The bogie 12 has plural wheels 13 which run on a track 16. The body 11 is mounted on the bogies 12.
FIG. 7 is a plan view showing a constitution of the bogie 12. In FIG. 7, a pair of wheels 13 are fixed on an axle 19. The axle 19 is borne by a pair of bearings 20 at both ends thereof. The bearings 20 are movably held on a frame 18 by way of springs (not shown). Thereby, the frame 18 is supported by the wheels 13. A smaller gear 23 is fixed on an end of a rotation shaft 22 of a traction motor 21. A larger gear 24 is fixed on the axle 19 and engaged with the smaller gear 23. The traction motor 21 is hung on the axle 19 by a pair of bearings 25 and a nose 26 of the traction motor 21 is suspended on the frame 18, elastically.
In the electric locomotive 10, the traction motor 21 is high-powered and it is relatively large. As shown in FIG. 7, the high-powered traction motor 21 must be contained in a small space between the pair of wheels 13. Therefore, it is requested to make a constitution of a gear drive device simple and occupied space of the gear drive device small. Accordingly, the above-mentioned constitution, wherein the traction motor 21 is directly hung on the axle 19 and a driving force of the traction motor 21 is transmitted to the axle 19 by a gear train of the smaller gear 23 and larger gear 24 thereby occupying the relatively small space, is adopted as a drive device of the electric locomotive 10. Such a drive device is called "Nose suspended/axle hung drive device".
Weight of the locomotive 10, which is mainly the sum of the weights of the body 11 and the frame 18 of the bogie 12, acts on the bearings 20, distribusively. Reaction forces act on parts of the axle 19 where the wheels 13 are fixed. Therefore, the axle 19 is deflected. The deflection of the axle by the weight of the locomotive 10 is shown in FIG. 8. As shown in FIG. 8, loads "W" due to the weight of the body 11 and the frame 18 act on points 20a which are pressure cone apexes of the bearings 20 on the frame 18. And the reaction forces act on points 13a which are the fixing point of the wheels 13 on the axle 19. As a result, the axle 19 is deflected as shown by deflection curve 19b from a virtual line 19a. The virtual line 19a is a center axis of the axle 19 when no load is applied thereto. The larger gear 24 which is fixed on the axle 19 inclines vertical to the deflection curve 19b cased by the deflection of the axle 19 and the angle of deflection of the larger gear 24 between the center axis 24a of the larger gear 24 and the virtual line 19 a is designated by ".alpha.".
Next, the smaller gear 23 which is fixed on the rotation shaft 22 of the traction motor 21 is considered. FIG. 9(A) is a cross-sectional side view showing a constitution of the traction motor 21. In FIG. 9(A), the traction motor 21 comprises a stator 27, a rotor 28 which is fixed on the rotation shaft 22 and bearings 29 for rotatively bearing the rotation shaft 22 on a housing 30. As shown in FIG. 7, a driving force of the traction motor 21 is transmitted from the smaller gear 23 to the larger gear 24. Hereupon, direction of a reaction force from the larger gear 24 to the smaller gear 23 changes responding to the rotation direction of the traction motor 21.
When the motor is seen in a direction shown by arrow X in FIG. 9(A), when the rotation shaft 22 of the traction motor 21 is rotated in clockwise direction, the smaller gear 23 receives a reaction force in a direction shown by arrow P.sub.1 in FIG. 9(B) from the larger gear 24. Therefore, the rotation shaft 22 of the traction motor 21 is distorted as shown by deflection curve 22b in FIG. 9(B). The center axis 23a of the smaller gear 23 is inclined against a virtual line 22a and the angle of deflection is designated by ".beta..sub.1 ".
On the other hand, when the rotation shaft 22 of the traction motor 21 is rotated in counterclockwise direction, the smaller gear 23 receives the reaction force in opposite direction shown by arrow P.sub.2 in FIG. 9(C) from the larger gear 24. The rotation shaft 22 of the traction motor 21 is deflected as shown by deflection curve 22b in FIG. 9(C) wherein the angle of deflection of the shaft 22 is designated by ".beta..sub.2 ".
FIG. 10(A) is cross-sectional view showing a relative inclination between tooth trace of the smaller gear 23 shown in FIG. 9(B) and that of the larger gear 24 shown in FIG. 8, in case that the traction motor 21 rotates clockwise seen from the direction shown by arrow X in FIG. 9(A). In FIG. 10(A), numeral 23b designates a cross-section of a tooth of the smaller gear 23, which is engaged with the larger gear 24, on a pitch circle. Numeral 24b designates a cross-section of a tooth of the larger gear 24, which is engaged with the smaller gear 23, on a pitch circle. In FIG. 10(A), an angle designated by ".alpha." is the same as the angle of deflection of the larger gear 24 shown in FIG. 8, an angle designated by ".alpha..sub.1 " is the same as the angle of deflection of the smaller gear 23 shown in FIG. 9(B), and an angle designated by ".theta..sub.1 " is an angle of relative inclination between the tooth trace of the tooth 23b of the smaller gear 23 and the tooth trace of the tooth 24b of the larger gears 24, which are engaged with each other.
FIG. 10(B) is a cross-sectional view showing a relative inclination between tooth trace of the smaller gear 23 shown in FIG. 9(C) and that of the larger gear 24 shown in FIG. 8, in case that the traction motor 21 rotates in counterclockwise direction seen from the direction shown by arrow X in FIG. 9(A). In FIG. 10(B), the teeth 23b and 24b and an angle designated by ".alpha." are similar to those in FIG. 10(A). An angle designated by ".beta..sub.2 " is the same as the angle of deflection of the smaller gear 23 in FIG. 9(C) and an angle designated by ".theta..sub.2 " is an angle of relative inclination between the tooth trace of the tooth 23b of the smaller gear 23 and the tooth trace of the tooth 24b of the larger gear 24, which are engaged with each other.
As shown in FIGS. 10(A) and 10(B), different angles of deflection occur in the smaller gear 23 and the larger gear 24, which are engaged with each other for transmitting a rotation force of the traction motor 21, corresponding to the rigidities of the rotation shaft 22 and the axle 19 whereon the gears 23 and 24 are fixed, respectively. Therefore, if the smaller and larger gears 23 and 24 are general spur gears, the teeth 23b and 24b contact at an end point designated by "A" in FIG. 10(A) or an end point designated by "B" in FIG. 10(B). Such a phenomenon is very disadvantageous in view of the durability of the teeth of the gears 23 and 24.
On the other hand, as shown in FIG. 11 which is a cross-sectional view showing another conventional tooth traces, teeth of one of the smaller gear 23 and the larger gear 24 (generally the smaller gear 23) have roundish section R.sub.c11 on the pitch circle, for preventing the above-mentioned disadvantage. This rounding of section is called "crowning", whereby the contacting point of the teeth 23c and 24b shifts to a point designated by "C" which is inside in comparison with the point designated by "A" in FIG. 10(A) or "B" in FIG. 10(B).
In the conventional gear drive device, axes of the gears, which are engaged with each other, are presupposed to be parallel under such a condition that no load is applied to them. As shown in FIG. 12, which is a cross-sectional view showing the shape of the tooth 23c of the smaller gear 23, the crowning is formed on both surfaces of the tooth 23c of the smaller gear 23. The tooth 23c whereto the crowning is provided has to satisfy the severest condition, for example, shown in FIG. 10(B). In the severest condition shown in FIG. 10(B), the relative inclination angle .theta..sub.2 of the gears 23 and 24 is larger than that of .theta..sub.1 shown in FIG. 10(A). Furthermore, for making the smaller gear 23 simple, teeth of the smaller gear 23 are generally formed symmetrical with respect to radii on a sectional plane vertical to the rotation axis. Therefore, radius of curvature R.sub.c11 of the crowning is unified to the smaller one.
Generally, a reaction force at a contacting point, when barrel shaped bodies contact each other as shown in FIG. 13, is approximated by a following equation (1). ##EQU1## Hereupon, .sigma.: tangential contact stress at a point "C" in FIG. 13,
P: pressing force acting between the barrel shaped bodies 23d and 24d in FIG. 13, PA1 K: a constant including Young's moduli and Poisson's ratios of materials of the barrel shaped bodies 23d and 24d in FIG. 13, PA1 R.sub.1 : a radius of the barrel shaped body 23d in a section including the point "C" and vertical to the axis thereof in FIG. 13, PA1 R.sub.2 : a radius of the barrel shaped body 24d in a section including the point "C" and vertical to the axis thereof in FIG. 13, PA1 R.sub.c1 : a radius of the barrel shaped body 23d in a section including the point "C" and parallel to the axis thereof in FIG. 13, PA1 R.sub.c2 : a radius of the barrel shaped body 24d in a section including the point "C" and parallel to the axis thereof in FIG. 13.
In FIG. 13 and the above-mentioned equation (1), R.sub.1 corresponds to a radius of an involute tooth of the smaller gear 23; R.sub.2 corresponds to a radius of an involute tooth of the larger gear 24; R.sub.c1 corresponds to the radius of the crowning R.sub.c11 of a tooth surface of the smaller gear 23; R.sub.c2 corresponds to a radius of the radius of the crowning of a tooth surface of the larger gear 24 (which is actually infinity since the crowning is not put in practice); the pressure "P" corresponds to an operating force of the gears 23 and 24, and therefore the contact stress ".sigma." corresponds to a contact stress on teeth surfaces of the gears 23 and 24. From the equation (1), it is known that the larger the radiuses R.sub.1, R.sub.2, R.sub.c1, R.sub.c2 are made, the smaller the contact stress ".sigma." can be made.
As shown in FIGS. 11 and 12, in the conventional gear drive device, the radius R.sub.c11 of the crowning must be decided so as to respond to the larger relative inclination angle .theta..sub.2. Therefore, the radius R.sub.c11 of the crowning becomes smaller. As a result, the contact pressure of the teeth of the gears becomes larger.